1. Field of the Invention
This invention relates to the field of orthopedic surgery and, particularly to modular screws having a polyaxial head.
2. Description of the Prior Art
It is conventional to install surgical appliances, such as rods or plates, to correct skeletal deformities and injuries in which the appliance spans several vertebrae. The surgical appliance is secured to the vertebrae and maintains a stable spatial relationship between them. Pedicle screws are inserted into the vertebrae to anchor the appliance in place. Because of the non-uniform anatomy of the vertebrae, the screws are usually misaligned relative to each other. To compensate for these variances, the rods may be bent to match the location of the screws or the screws may have connectors capable of angular displacement to engage the rods or both.
For example, U.S. Pat. No. 6,669,697 to Pisharodi teaches the use of eccentric shims between the bone screws and the surgical appliance to correct the angle between the rod and the screw. Shluzas, U.S. Pat. No. 6,648,888, teaches a bone screw with a spherical head rotating in a tapered connector. The connector has opposing tabs to capture the spinal rod and a set screw to hold the rod in the yoke. Reissued patent, RE37,665, teaches another spherical headed screw with a swiveling connector having an upstanding yoke capturing a spinal rod. Varieur, U.S. Pat. No. 6,641,586, teaches another spherical headed bone screw and an upstanding yoke retainer for the surgical appliance. The retainer has an internal nut and an external ring threaded onto the yoke to secure the appliance. These devices all have a multitude of small parts that must be assembled at the surgical site.